What's the Buzz? Rowdy Teenagers Don't Want to Hear It - New York Times:
Now 39, Mr. Stapleton has taken the lesson he learned that day - that children can hear sounds at higher frequencies than adults can - to fashion a novel device that he hopes will provide a solution to the eternal problem of obstreperous teenagers who hang around outside stores and cause trouble.
The device, called the Mosquito ("It's small and annoying," Mr. Stapleton said), emits a high-frequency pulsing sound that, he says, can be heard by most people younger than 20 and almost no one older than 30. The sound is designed to so irritate young people that after several minutes, they cannot stand it and go away.